Prealloyed ferrous powders suitable for molding without other powders by conventional powder metallurgy techniques have proceeded from the earlier usage of large amounts of alloying elements to small but balanced amounts of allowing ingredients to obtain equivalent and useful physical properties in comparison to wrought alloy steels. Major achievements in economy cannot be achieved because the balanced alloy ingredients are still too excessive in amount and the entire powder making cycle must be used for each distinct chemical composition. Thus, pre-alloyed powders are expensive compared to simple iron powders conventionally produced and it is unlikely that part producers will accept the limit number of pre-alloyed compositions commercially available.
Mechanical mixtures of simple iron powders with small amounts of pre-alloyed powders has been deemed a promising mode of providing alloying during sintering of the compacted powders, but exactly how to achieve adequate and economical homogenization of the ingredients of the alloy powder into the base iron powder is not known to the art. The prior art recognizes that, conceptually, admixtures seem to offer substantial economic advantages over pre-alloyed powders.
One method of admixing and joining master alloy and base iron powders is to use solid state particle diffusion; this is unsatisfactory because it is limited by the number of inner particle contacts. Another method of carrying out master alloy and base powder admixing and joining is to use gasification of one of the components to achieve diffusion; this is limited because of the absence of sufficient acceptable candidates or components for this method. However, if the master alloy powder is converted to a liquid phase there can occur an increase in particle contact. To arrive at this goal and to do so economically, there must be an improvement in the kinetics of the sintering process, particularly a reduction in the necessary liquidus temperature for the entire alloying powder during sintering.
This invention finds particular use for copper, and equivalent carbon diffusion barriers, to dramatically improve sintering kinetics. Copper has been used in powder metallurgy, not only as an alloying ingredient, but as an infiltrant to the compacted powders for preventing errosion of the surface. Heavy quantities of copper powder have been typically mixed with a fer uginous powder to provide infiltration. The mass, resulting from this processing, shrinks and warps considerably through coalescence thereby reducing surface contact between the infiltrant and the ferruginous mass. But this art, by itself, even though incorporating copper, does not teach how one can reduce the liquidus temperature of the master alloy powder to a eutectic temperature when combined with a low carbon base powder.
Some thought, unrelated to sintering kinetics, has been given by the prior art to coating a base iron powder with copper or other low melting equivalents. It was hoped that this would create a strong welded network between the base iron powder particles. Instead, this has resulted in a significant reduction of the physical properties of the resulting sintered product.